Generally, a spring assembly having free terminal convolutions (hereinafter, referred to as spring assembly) is an elastic body to provide a cushion and shock-absorbing function together with padding members each layered on an upper surface and a lower surface of the spring assembly. The spring assembly is composed of a plurality of coil springs arranged in a side-by-side arrangement, usually in parallel columns and parallel rows.
In the case where coil springs are regularly arranged in upper and lower directions of upper and lower border wires which form the upper and lower surfaces of the spring assembly, the terminal convolutions of the coil springs are exposed in a given height from an upper portion of the upper border wire and/or a lower portion of the lower border wire.
As shown in FIG. 1, the spring assembly, generally indicated at 100, includes an array of springs 110 arranged in parallel columns C and parallel rows R, upper and lower border wires 120 and 120′ disposed on upper and lower portions of the array of springs 110 for supporting the outermost coil springs 115, helical coils 130 engaged to the parallel row R of the array of springs 110 for fixing the terminal convolutions of the adjacent coil springs 115.
The coil springs 115 are arranged in a vertical direction, where they are fixed in a space between the upper and lower border wires 120 and 120′ by means of sheet metal clips 140. The coil springs 115 have free terminal convolutions 115a exposed outwardly from the upper and/or lower border wires 120 and 120′ at an upper end portion and/or a lower end portion of the body.
The coil springs 115 are fixed in the space between the upper and lower border wires 120 and 120′, and the helical coils 130 are engaged to the upper and lower portions of the spring array 110 in the row directions R, thereby forming the free terminal convolutions 115a on the upper portion of the upper border wire 120 and/or the lower portion of the lower border wire 120′.
In the case where the terminal convolutions 115a of the coil springs 115 are exposed outwardly from the upper and lower border wires 120 and 120′ on the basis of a transverse plane of the spring array 110, when the padding members 150, such as non-woven fabric or floss silk, are layered on the upper surface and the lower surface of the spring array 110 and are enclosed by a cover, as shown in FIG. 2, the padding member 150 and the terminal convolutions 115a are in contact with each other, thereby causing the padding member to be damaged.
In addition, if the terminal convolutions 115a are deflected to one direction by the external force applied to the mattress, the spring assembly 100 has a problem in that the resilient force of the coil spring 115 is deteriorated due to the load focused toward one point. The deflected terminal convolutions 115a are contacted with a periphery surface of the upper border wire 120 or the lower border wire 120′, thereby producing a frictional noise.